Obviously don't REALLY think there is anything wrong with having kids.
And Mai is indeed super-adorable.
My issue is mostly due to the age difference making it kinda icky. I guess some people like to have kids pretty young, but seems like a bit more baggage than I can imagine your average 17/18 year old dealing with all that well. If Saki wasn't still in high school, I wouldn't have much of an issue with it.
He also seems like a pretty awful parent (the yelling and the "almost" physical abuse). That specific scene just left a bad taste in my mouth. I get the feeling Mai might be better off living with her grand-parents, but, hey, maybe he pulls himself together.